This invention relates to electrostatically aided atomization and coating of articles with charged particles. It is disclosed in the context of certain types of coating material dispensers. However, it is believed to be useful in a wide range of coating dispensing applications. As used in this application, terms such as "electrically conductive" and "electrically non-insulative" refer to a broad range of conductivities electrically more conductive than materials described as "electrically non-conductive" and "electrically insulative." Terms such as "electrically semiconductive" refer to a broad range of conductivities between electrically conductive and electrically non-conductive.
In its early years, the field of electrostatically aided coating material atomization and dispensing was dominated by the dispensing of coating materials containing organic solvents. These solvents and the coating materials they carried typically were electrically non-conductive or only very slightly conductive, but the carriers or solvents were also relatively volatile. The particles of these coating materials thus could ordinarily be charged by contact with, or at least passage within relatively short distances of, electrodes maintained at relatively high magnitude potentials with respect to the article(s) to be coated by the atomized coating material particles. However, care needed to be taken not to stimulate high energy electrical discharge across the space between the electrodes and the article(s) being coated. This need dictated considerable attention by operators of such equipment. The volatility of these solvents also raised environmental concerns about the release of so-called voc's (volatile organic compounds).
Efforts have continued to enhance solvent based coating systems, both against the hazards associated with having relatively high magnitude electrical potentials across atmospheres containing voc's, and against the inevitable close proximity of operators to the highly charged electrodes of such equipment. Standards for testing such equipment have been promulgated by a number of testing agencies in various countries. Illustrative of such standards is the Electrostatic Finishing Equipment Approval Standard, Class Number 7260, promulgated by Factory Mutual Research Corporation (the FM standard).
The FM standard includes protocols for the testing of both manual equipment (for example, hand held coating atomizing and dispensing guns--the FM standard, chapter 5) and automatic equipment (for example, atomizers mounted on robot arms--the FM standard, chapter 6). Among the tests in both cases is a test in which the equipment at operating voltage is probed using a grounded metal sphere having a diameter of one inch (about 2.5 cm). This test takes place in an explosive atmosphere of propane in air. An explosion is a failed test. To achieve FM approval, the equipment must, inter alia, pass this test. The FM standard has caused considerable research and improvement in the safety of electrostatic coating systems. Some ways in which the protocols can be addressed are illustrated and described in co-pending U.S. Ser. No. 08/955,039 filed Oct. 21, 1997, titled SAFE CHARGING, and co-pending U.S. Ser. No. 09/046,383 filed Mar. 23, 1997, titled SAFE CHARGING WITH NON-INSULATIVE ATOMIZER, both assigned to the same assignee as this application.
In atomizers constructed generally as described in U.S. Pat. Nos. 5,622,563; 5,633,306; and, 5,662,278, illustrated in FIGS. 1a-b, the atomizer 8 is constructed with a relatively well-defined atomizing edge 10. Referring specifically now to FIG. 1a, the semiconductive coating 12 applied to the rearward, or outer, surface 14 of the atomizer 8 extends all the way to edge 10, increasing the likelihood of electrical contact between the coating 12 and the coating material 16 being atomized from edge 10. This contact, of course, increases the likelihood that the coating material 16 being atomized from edge 10 will be electrically charged and will be attracted to the article to be coated thereby, all in accordance with known principles.
Referring now particularly to FIG. 1b, however, what sometimes happens to atomizer 8 as it is used can be seen. The abrasive nature of some coating materials 16, poor maintenance habits, and other factors can lead to a reduction in the sharpness of edge 10, cause rounding of edge 10, and cause the semiconductive coating 12 to wear away from edge 10. This phenomenon is accelerated somewhat as the edge wears round, owing, it is believed, to the surface tension of the coating material causing the coating material to migrate back along the lip of the atomizer 8 toward the semiconductive coating 12. Because the coating material remains uncharged until it contacts the semiconductive coating 12, there is less tendency for the coating material to leave the lip. As the coating material 16 flows to edge 10 to be atomized, it becomes less likely that the coating material will contact the semiconductive coating 12. It therefore becomes less likely that the coating material will be electrically charged as it is atomized from edge 10. This manifests itself in a reduction in transfer efficiency, the ratio of the amount of coating material being deposited on the article to be coated to the amount of coating material dispensed by the atomizer 8.